[Discuss-gnuradio] User questions answered

[Matt Ettus]

>  I guess I'm confused about what -f, -c and -i really do in
> usrp_siggen.py?

There are 3 different "frequencies" here.  First, -f refers to the
frequency of the waveform which software is generating and sending over
the USB bus think of this as a first (low) IF.  The -c refers to the
digital upconverter frequency (DUC).  This is the frequency which the
USRP hardware translates the first signal to.

Lastly, you have -i which controls the USRP interpolation rate.  USRP
output samples are at 128 MS/s, so this is the factor by which your data
is upsampled in the USRP hardware.  If you use 128, it means that your
PC must send 1 MS/s across the bus so that the USRP can upsample by 128
to make 128 MS/s.  This number must be a multiple of 4.

> Also, what does the scrolling
> text UuUuUuUuUuUu mean?  Seems to work even if the text does not appear.

The "u" stands for USRP, and the "U" for "underrun" meaning your PC is
not generating data fast enough to keep the USRP running.  Sometimes you
may also see uO, meaning USRP overrun.  This happens on receive
applications, where your PC is not reading out the data FROM the USRP
fast enough, and you end up dropping data because your buffers aren't
big enough.  The same things apply with aU and aO, but for audio devices.

Every time you see uU the USRP has nothing to transmit when it needs
data, so it sends zeros.


Matt